1. Field of the Invention
The present invention is directed to the field of passive solar collectors, including improvements in the support for the reflective surface of the collector and methods of making, improvements in the receiver element, and controls for moving the collector to track the apparent trajectory of the sun.
2. Description of Related Art
The development of solar technology promises to reduce the damage to the earth's environment brought about by the extraction and burning of fossil fuels. Accordingly, increased attention has been devoted recently to ways to make this technology more efficient and economical.
Solar energy is conventionally harnessed in one of two ways: with a photovoltaic device, in which solar radiation generates electric current directly; or with a passive solar collector, in which solar radiation is incident on a receiver carrying a heat transfer fluid. The heat transfer fluid may then be used to drive a turbine, or other power generating device.
Passive solar power generation facilities have been built on a large scale in the United States, wherein heated water drives turbines generating power in the 10s to 100s MW range per facility. However, the large scale and technological complexity of these apparatus has hampered their development. In particular, it has not generally been possible to expand this technology downward, into applications where less power is generated, or where the heated water is used for heat or hot water directly, as opposed to for generating power. The glass reflectors and complex tracking mechanisms used in these power generating facilities are prohibitively expensive for the smaller end use.
There are also commercial solar thermal energy systems operated for medium temperature usage, in a range of about 80° C. to about 250° C. Typically, these are flat collectors set on a fixed inclination facing south.
A more efficient solution is to provide a focusing reflective surface in the shape of a parabola, so that the sun's rays are concentrated on a receiver positioned in the focal line of the parabola carrying a heat transfer fluid. The collector pivots to track the apparent movement of the sun from East to West.
One problem with the focusing collector systems in the past has been the difficulty of forming and calibrating a reflective surface in the required parabolic shape. The curved surface of the reflector in these apparatus must be precisely calibrated, which requires skill and training.
U.S. Pat. No. 5,069,540 proposes a method for making a solar collector from a mold using an amorphous, hard-curing material, such as stucco or cement, to address the problem of maintaining a surface with specified dimensions. U.S. Patent Application Publication No. 2008/0078380 describes a complicated system of brackets designed to support reflector panels at a given curvature. Neither of these systems provides a reliable, inexpensive system for providing a reflector with a desired curved surface.
Thus, there is a continued and increasing need for systems wherein the curvature and position of the reflector can be rapidly and reliably determined.
Conventionally, the receiver element of a passive solar collector has been encased in an evacuated glass tube, such as disclosed in U.S. Patent Application Publication No. 2008/0078380. Such devices improve collector efficiency by reducing the convective heat loss from the receiver to the surrounding environment, but such systems are complex, costly, and difficult to maintain. Thus, there is a need in this art for a receiver element that is efficient without requiring a vacuum tube.
A solar collector works most efficiently if it is pointed at the sun. Although various tracking systems are known, some of which may be adapted for use with a solar collector, there continues to be a need in the art for a tracking system that points the collector at the sun so that the maximum amount of solar radiation is collected, and such that temporary cloud cover, night fall, and other low-sunlight incidents do not cause a failure of the system to accurately track the apparent trajectory of the sun.